By using techniques that help to reconstruct past climates, and by tracking trends in the present, we can predict how current climates might change. Overall, the world is warming, yet, as we are still in an ice age, eventually the current interglacial period should end, allowing glaciers to advance towards the equator again (although likely not for about 100,000 years). However, because the Earth is already getting warmer, the effects of anthropogenic warming are amplified through feedback. Some scientists worry that, if not curbed, human activity could actually disrupt the cycle and knock the planet entirely out of the interglacial period, melting all the ice on Earth.

Causes of Change

While astronomical and tectonic forces will continue to cause climatic shifts, they act so slowly that they will be overshadowed in the near term by humaninduced effects. In 1956, NOAA established the Mauna Loa Observatory (MLO) in Hawai‘i to measure a variety of atmospheric parameters, including carbon dioxide (CO₂) concentration. The CO₂ record extends from 1958 to present, and it shows the influence of both natural and anthropogenic processes (Figure 8.12). The zigzag pattern is the result of seasonal photosynthesis in the Northern Hemisphere. In spring and summer, the growth and increased photosynthetic activity of plants draws CO₂ out of the atmosphere. Conversely, it accumulates in the atmosphere during fall and winter when plants are dormant. The overall upward trend is caused by human activity. Industrialization, fossil fuel combustion, and deforestation all contribute CO₂ to the atmosphere, adding it at a rate much faster than natural processes can remove it. Analyses of ancient atmosphere samples preserved in glacial ice cores show CO₂ levels to have been 180 parts per million (ppm) at the height of the last ice age and 280 ppm at its end. The amount of CO₂ in the atmosphere has been increasing at a rapid rate since the start of the industrial revolution, and it has accelerated since the end of World War II. In May 2013, measurements at MLO reached 400 ppm CO₂ for the first time.

Figure 8.12: Measured concentration of atmospheric carbon dioxide (1958 to present) at MLO.

While some atmospheric carbon dioxide is necessary to keep Earth warm enough to be a habitable planet, the unprecedentedly rapid input of CO₂ to the atmosphere by human beings is cause for concern. Everything we know about atmospheric physics and chemistry tells us that increased CO₂ leads to a warmer planet. Multiple paleoclimate data sets verify this conclusion, and modern measurements confirm that we are living in an increasingly warmer world. The increasing heat is causing glaciers and sea ice around the globe to melt, and as the ground and ocean they covered is exposed, these darker surfaces absorb and re-radiate increasing amounts of heat.

As permafrost in high latitudes melts, carbon in the soil becomes free to enter the atmosphere and, worse, buried organic material can be converted by bacteria into the even more potent greenhouse gas methane. Less directly, higher temperatures lead to more frequent and severe droughts, which, in turn, lead to more wildfires that release carbon and aerosols into the atmosphere. Aerosols can have a cooling effect since they reflect away radiation from the sun, but they can also pose a public health hazard.

Water is extremely good at absorbing heat: water vapor is actually the most effective greenhouse gas. Higher temperatures increase evaporation and allow the air to retain more water. While water vapor feedback is the most significant reinforcer of climate warming, water tends to move out of the atmosphere in a matter of weeks—other greenhouse gases, such as carbon dioxide and methane, linger in the atmosphere for years.

The Southwest contributes significantly to climate change. The population of any industrialized and particularly wealthy country produces pollution; the majority of these emissions come from the use of petroleum. The more than 16 million residents of the Southwest use carbon-rich fossil fuels to provide electricity for lighting, cooling, and appliances, to fuel their transportation and industry, and to make the products they use. Burning those fossil fuels releases carbon into the atmosphere, which warms the Earth. Of the Southwestern states, Arizona emits the most greenhouse gases, releasing 94 million metric tons of carbon dioxide per year. Although this pales in comparison to emissions from the nation’s highest CO₂ producer—Texas, which releases nearly 656 million metric tons of CO₂ per year—Arizona’s greenhouse gas emissions are rising rapidly compared with the nation as a whole. In the last decade, the United States has decreased the total amount of energy-related carbon dioxide emissions by almost ten percent, yet Arizona’s emissions have increased by 9% thanks to a growing population that relies heavily on oil and natural gas for energy. Emissions from Colorado and Utah have also increased over the past decade, growing 7% and 2%, respectively.

On the other hand, Southwestern states are making changes to reduce human impact on the climate. New Mexico has reduced its CO₂ emissions by more than four metric tons in the last decade. The cities of Aspen and Lafayette, Colorado, as well as the state of New Mexico, were early adopters of the 2030 Challenge, an effort to reduce fossil fuel use in buildings so that both new and renovated buildings would qualify as carbon neutral by the year 2030. Additionally, states are beginning to step up their use and production of renewable energy. As of 2015, Arizona ranks 30th in the nation for renewable energy production, much of which it produces from hydroelectricity and biomass.

Trends and Predictions

Studies show that the Southwest’s climate is changing right now, and that change has accelerated in the latter part of the 20th century. These changes include the following:

• The number of days with temperatures above 35°C (95°F) and nights above 24°C (75°F) has been steadily increasing since 1970, and the warming is projected to continue (Figures 8.13 and 8.14).
• The onset of stream flows from melting snow in Colorado has shifted two weeks earlier due to warming spring temperatures. Flows in late summer are correspondingly reduced, leading to extra pressure on the state’s water supplies.
• Streamflow totals for the last decade in the Great Basin, Rio Grande, and Colorado River were between 5% and 37% lower than their 20thcentury averages.
• Since 1980, tree mortality in forests and woodlands across the Southwest has been higher and more extensive than at any time during the previous 90-year record; this is attributed to higher temperatures, drought, and the eruption of bark beetles that are able to survive through warmer winter weather.
• Increased heat in the Pacific Ocean has altered the weather patterns of Pacific storms, decreasing snowfall in the mountains of western Utah and Arizona.
• In the last decade, the Southwest's frost-free season has increased by approximately 7% compared to the average season length for the 20th century.
• The seasonality and transmission frequency of insect-borne diseases and other infectious diseases prevalent in the Southwest, including plague, valley fever, and Hanta, are influenced by warming trends.

Recent warming within the Southwest has been among the most rapid in the United States, and models predict that the area's climate will continue to warm. The average annual temperature in most of the Southwest is predicted to rise 2.2° to 5.5°C (4° to 10°F) by 2100. Summer heat waves will become hotter and longer, while winter cold snaps will occur less often. These increased temperatures lead to a whole host of other effects, including a decrease in snowpack, declines in river flow, drier soils from more evaporation, and the increased likelihood of drought and fires. In winter, rising temperatures have increased the amount of frost-free days—today, most of the Southwest experiences about 17 fewer freezing days than it did over the last century. By 2070, one can expect up to 38 more days of freeze-free weather each year (Figure 8.15). These warmer temperatures and increased precipitation have helped bring on longer growing seasons. While changes in the growing season can have a positive effect on some crops (such as melons and sweet potatoes), altered flowering patterns due to more frost-free days can lead to early bud bursts, damaging perennial crops such as nuts and stone fruits.

Figure 8.13: Global temperature change since the 1880s. The Earth's average surface temperature has progressively risen over the last five decades.

Figure 8.14: Projected temperature increases for the Southwestern states over the next century, as compared to the average for 1971–1999. The "higher emissions" scenario assumes emissions continue to rise, while the "lower emissions" scenario assumes a substantial reduction in emissions. In both cases, temperatures will continue to rise.

Warmer temperatures also make it easier for insect pests to overwinter and produce more generations. Bark beetles, which normally die in cold weather, have been able to survive through the winter and reproduce, increasing tree mortality. For example, high winter temperatures between 2000 and 2003 correlated to bark beetle outbreaks that devastated pinyon pine throughout the Southwest, leading to nearly 90% mortality at some sites in Colorado and Arizona. As of 2010, bark beetles in Arizona and New Mexico have affected more than twice the forest area burned by wildfires in those states.

Water supply is an important issue in the Southwest, and communities will need to adapt to changes in precipitation, snowmelt, and runoff as the climate changes. Agriculture accounts for more than half of the Southwest’s water use, so any major reduction in the availability of water resources will create a serious strain on ecosystems and populations. Drier days and higher temperatures will amplify evaporation, increasing the desertification of already arid areas and affecting natural ecosystems as well as increasing pressure on the water supply for agriculture and cities (Figure 8.16). An increased frost-free season length also leads to increased water demands for agriculture and heat stress on plants. Cattle ranches throughout the Southwestern states rely on rain-fed grazing forage, making them extremely susceptible to climate change and drought. In addition, temperature increases and recent drought lead to earlier spring snowmelt and decreased snow cover on the lower slopes of high mountains, bringing about more rapid runoff and increased flooding. These changes to rain and snow-pack are already stressing water sources and affecting agriculture. Precipitation has become more variable from year to year, and heavy downpours across the US have increased in the last 20 years. Because higher temperatures mean greater evaporation and warmer air can hold more water, precipitation will occur in greater amounts at a time, but less frequently. Although there has so far been little regional change in the Southwest’s annual precipitation, the area’s average precipitation is expected to decrease in the south and remain stable or increase in the north. Most models predict a decrease in winter and spring precipitation by the middle of the century, and more frequent precipitation extremes during the last half of the century.

Figure 8.15: Projected frost-free days for the Southwestern states over the next century, as compared to the average for 1971–2000. The “higher emissions” scenario assumes emissions continue to rise, while the “lower emissions” scenario assumes a substantial reduction in emissions. Gray areas are projected to experience more than 10 frost-free years.

Figure 8.16: Projected 21st-century supply-demand imbalance for the use of water from the Colorado River. The Colorado drains roughly 15% of the continental United States, and is relied upon for municipal and agricultural use by over 35 million people in seven states.

The causes of specific weather events such as tornados and severe thunderstorms are incredibly complex, although climate change has enhanced some correlated factors, such as increased wind speed and an unstable atmosphere. Higher atmospheric moisture content has also been correlated with an increased incidence of tornados and winter storms. However, although climate change is predicted to enhance the intensity of severe weather, there is currently no way to calculate what effect climate change will have on the frequency of specific storm events—for example, we might see more powerful tornados, but we do not know if we will see more of them.

All over the Southwestern US, residents and communities have begun to adapt to climate change, and to plan for future changes that are expected to come.